The Concept of Child Themes

WordPress, by default, supports child themes. Child themes look exactly like their parent themes, unless you make modifications to them. Child themes inherit all the templates, functions, and CSS of parent themes.This compartmentalizes variations so that you can make changes without affecting the integrity of the parent theme.

A child theme is usually contained in a folder having a styles.css (required) and afunctions.php file. functions.php is not mandatory, but you will need it if you want to include some custom functions on top of your parent theme (which, in this scenario, is the WordPress theme framework). Both the child theme and parent theme folders will be in the themes directory of your WordPress installation.

You can override the inherited traits from its parent by modifying its own styles.css andfunctions.php files.

Here is how you declare a child theme (this goes inside styles.css, at the top).